1. Field of the Invention
The present invention is generally related to vibration monitoring and is more specifically concerned with vibration monitoring of cutting tools used for high-performance machining.
2. Description of the Related Art
Driven by cost reduction, high productivity, throughput and quality goals, the machining industry has an increasing need for improved characterization of cutting tools and for controlling vibration and chatter during the machining process. If uncontrolled, vibration and chatter can reduce surface finish quality, limit dimensional accuracy, increase tool wear and create high levels of noise. These undesirable performance attributes can, in turn, lead to increased machine wear, reduced throughput and higher scrap rates. Furthermore, there are growing demands for high-speed milling, for the use of small tools, and for machining advanced materials with poor machining characteristics, all of which exacerbate vibrational problems. Conventional approaches for control of vibration and chatter, such as increasing the stiffness of the tool and reducing the cutting depth or machine speed, clearly reduce production throughput and are no longer satisfactory.
Recently, more sophisticated approaches that take advantage of modern dynamic models and improved sensors and controls have been developed. These approaches typically utilize knowledge of the mass, stiffness, and vibrational and damping characteristics of the tool to estimate the cutting depth limits and the optimal spindle speed. Only limited control has been attained since prior art sensors permit dynamic characterization of the tool only prior to the machining operation. Optimal control requires that vibration be sensed as close as possible to the tool tip during the actual machining process since tool vibration and chatter are significantly affected by interaction between the tool and the workpiece.
One prior art method for predicting vibrational characteristics of a cutting tool involves exciting the static tool with an impulse hammer and detecting the resulting vibrations via an accelerometer mounted at or near the tip of the tool. The force of the impulse hammer excitation is usually chosen to simulate the force expected during the cutting process, which typically ranges from tens of Newtons for small tools up to a few thousand Newtons for larger tools. Nonetheless, the impulse hammer method does not adequately predict vibration during the machining process since accelerometers cannot be used to detect vibration of a rotating tool. A further limitation of the impulse hammer method is that accelerometers are typically too large and/or too heavy for use with the small tools often employed by mold manufacturers and the rapid prototype industry, for which the tool diameter may be as small as 1/64th inch. Strain gages have been adapted for sensing vibration but must also be mounted on the tool and cannot be used during tool rotation.
For dynamic vibrational characterization of a rotating tool, a non-contact sensor is required. Prior art methods for detecting cutting tool vibrations have employed a number of non-contact sensors, including capacitive probes, inductive probes, laser Doppler vibrometers, and acoustic pickup devices (e.g., microphones). An alternative prior art method involves detection of tool vibration from an indirect measurement, via measured fluctuations in tool rotation speed or spindle power requirements, for example.
There is no commercially available vibration sensor that provides the capabilities needed for advanced machining. These capabilities include dynamic tool vibration monitoring before and during the machining process, high measurement sensitivity and accuracy, insensitivity to the machining environment, minimal mechanical loading, compactness and low-cost. It would also be very beneficial to be able to monitor vibration of the workpiece, which may be appreciable for advanced materials that are difficult to machine. The availability of an accurate, robust and low-cost sensor to measure tool vibrational characteristics would be of great practical value.
In principle, a laser beam could be used to remotely detect vibration of a rotating tool via measurements of displacement of the tool surface as a function of time. However, such measurements are strongly influenced by the surface roughness of the tool, which greatly limits the utility of the method. Ultrasonic reflection measurements are much less sensitive to surface roughness of the target and have been shown to be useful for detecting vibration of static parts immersed in a liquid in a tank [D. Royer and O. Casula, Appl. Phys. Lett. 67(22), 27 (1995)]. It is not obvious how this approach could be applied to dynamic vibration monitoring of real-world cutting tools. Ultrasonic vibration measurements made in air are much less sensitive (compared to liquid phase measurements) due to scattering effects, and are significantly affected by turbulent flow of the hot air typically produced by friction during machining operations, as well as by mist and dust that are frequently present in the environment.